Laser oscillation apparatus

ABSTRACT

A laser oscillation apparatus, comprising a laser beam emitting unit for emitting an excitation light, an optical resonance unit at least having a laser crystal and an output mirror, a nonlinear optical medium which is placed in the optical resonance unit and which generates a second harmonic from a fundamental wave oscillated by the laser crystal, and a driving means for driving the laser beam emitting unit, wherein the nonlinear optical medium is held by a nonlinear optical medium holder, and the nonlinear optical medium holder can be rotated with respect to axes of at least two directions crossing to an optical axis passing through the optical resonance unit.

BACKGROUND OF THE INVENTION

The present invention relates to a laser oscillation apparatus using asemiconductor laser as an excitation source.

First, general features of a laser oscillation apparatus will bedescribed referring to FIG. 11.

FIG. 11 is a laser oscillation apparatus 1, schematically showing alaser oscillation apparatus using a second harmonic, and it comprises alaser light source 2, a condenser lens 3, a laser crystal 4, a nonlinearoptical medium 5, an output mirror 6, and laser driving means 9.

The laser light source 2, the condenser lens 3, the laser crystal 4, thenonlinear optical medium 5, and the output mirror 6 having a concavereflection surface are arranged on a same optical axis 10. A firstdielectric reflection film 7 is formed on a surface of the laser crystal4 facing to the condenser lens 3, and a second dielectric reflectionfilm 8 is formed on a concave reflection surface of the output mirror 6facing to the nonlinear optical medium 5.

The laser light source 2 generates a laser beam. In the presentembodiment, a semiconductor laser is used, and the laser light source 2has a function as a pump light generator for generating a fundamentalwave. The laser light source 2 is driven by the laser driving means 9,and the laser driving means 9 can drive the laser light source 2 bypulse driving.

The laser crystal 4 is a medium of negative temperature and it is usedfor amplification of the light. As the laser crystal 4, YAG(yttrium—aluminum—garnet), etc. doped with Nd³⁺ ions is adopted. YAG hasoscillation lines of 946 nm, 1064 nm, 1319 nm, etc. In addition to YAG,Nd with oscillation line at 1064 nm (YVO₄), or Ti with oscillation linesat 700 to 900 nm (Sapphire), etc. is used as the laser crystal 4.

The first dielectric reflection film 7 is highly transmissive to thelaser light source 2 and is highly reflective to an oscillationwavelength of the laser crystal 4. It is also highly reflective to asecond harmonic. The second dielectric reflection film 8 is highlyreflective to the oscillation wavelength of the laser crystal 4 and ishighly transmissive to the second harmonic.

As described above, it may be designed in such manner that the lasercrystal 4 is combined with the output mirror 6, the laser beam from thelaser light source 2 enters through the condenser lens 3, and theentered laser beam is reflected between the first dielectric reflectionfilm 7 and the second dielectric reflection film 8 and is pumped to thelaser crystal 4. The laser beam can be confined for a long time betweenthe first dielectric reflection film 7 and the second dielectricreflection film 8 through the nonlinear optical medium 5. As a result,the laser beam can be resonated and amplified, and a laser beam of thesecond harmonic can be projected through the output mirror 6.

Brief description will be given on the nonlinear optical medium 5.

When an electric field is applied on a substance, electricalpolarization occurs. When the electric field is small, the polarizationis proportional to the electric field. However, in case of strongcoherent light such as the laser beam, proportional relationship betweenthe electric field and the polarization is impaired, and nonlinearpolarization components proportional to square or cube of the electricfield become prominent.

Therefore, in the nonlinear optical medium 5, the polarization generatedby the laser beam contains a component proportional to square of thelight wave electric field. By the nonlinear polarization, bonding occursbetween light waves with different frequencies, and a second harmonic todouble the light frequency is generated. The generation of the secondharmonic is generally called “SHG (second harmonic generation)”.

In the conventional example as described above, the nonlinear opticalmedium 5 is placed in an optical resonance unit, which comprises thelaser crystal 4 and the output mirror 6, and this is called as internaltype SHG. Conversion output is proportional to square of fundamentalwave opto-electric power, and high light intensity in the opticalresonance unit can be directly utilized.

As the nonlinear optical medium 5, for instance, KTP (KTiOPO₄; titanylpotassium phosphate), BBO (β-BaB₂O₄; β-lithium borate), LBO (LiB₃O₅;lithium triborate), etc. are used. Primarily, it is converted from 1064nm to 532 nm.

KNbO₃ (potassium niobate), etc. is also adopted. Primarily, it isconverted from 946 nm to 473 nm. In FIG. 11, ω is an angular frequencyof the optical fundamental wave, and 2ω is an angular frequency of thesecond harmonic.

In the laser oscillation apparatus using the second harmonic, for thepurpose of generating a higher harmonic from the optical fundamentalwave oscillating in the optical resonance unit using a nonlinear crystal(KTP crystal), the following conditions are needed:

(1) Temperature control of the nonlinear crystal (phase coordinationtemperature at constant level of 25° C.)

(2) Phase coordinating conditions of the nonlinear crystal which issatisfied by adjusting a nonlinear crystal axis with respect to afundamental wave oscillation axis in the optical resonance unit.

Therefore, the laser oscillation apparatus of conventional type has acooling mechanism and an aligning mechanism of nonlinear crystal axis.

Referring to FIG. 12, description will be given now on the coolingmechanism and the aligning mechanism of the nonlinear crystal axis usedin the past.

On an optical resonator block 11 made of a material with high heattransfer property, a recessed portion 12 for accommodating a nonlinearoptical medium 5 is formed. An optical path hole 13 is provided, whichpasses through the recessed portion 12 and has an axis aligned with anoptical axis 10 of the laser oscillation apparatus. The optical pathhole 13 is cut by the recessed portion 12. A laser crystal 4 is disposedon a part of the optical path hole 13 closer to an incident side, and anoutput mirror 6 is provided on an exit side end of the optical path hole13. On the lower surface of the optical resonator block 11, a Peltierelement 14 is fixed.

The nonlinear optical medium 5 is placed on and closely fitted to thebottom surface of the recessed portion 12. The nonlinear optical medium5 is held at the lower end of an angle adjusting jig 15. The angleadjusting jig 15 has a knob 15 a extended in a direction of θ axis 16running perpendicularly to the optical axis 10. The knob 15 a isprotruded from the recessed portion 12, and an angle θ for the nonlinearoptical medium 5 can be adjusted around the θ axis 16 by the knob 15 a.

The non linear optical medium 5 is placed so that a nonlinear crystalaxis of the nonlinear optical medium 5 is aligned with the optical axis10. Because the nonlinear optical medium 5 is cut out along thenonlinear crystal axis, when the nonlinear optical medium 5 is closelyfitted to the bottom surface of the recessed portion 12, the position ofthe nonlinear crystal axis is determined within a horizontal plane withrespect to the optical axis 10. The nonlinear optical medium 5 isrotated by turning the knob 15 a while it is pressed against the bottomsurface of the recessed portion 12, and the angle θ is adjusted so thatthe optical axis 10 and the nonlinear crystal axis run in parallel toeach other within the same plane.

When the adjustment has been completed, the nonlinear optical medium 5is fixed on the optical resonator block 11 by adequate means such asbonding or by a screw.

An excitation light passing through the first dielectric reflection film7 is absorbed by the laser crystal 4. A fundamental wave oscillated bythe laser crystal 4 is reflected between the first dielectric reflectionfilm 7 and the second dielectric reflection film 8, and a secondharmonic generated from the nonlinear optical medium 5 is projectedthrough the output mirror 6.

As described above, the nonlinear optical medium 5 is a medium ofnegative temperature. In order to obtain predetermined stable secondharmonic output, it is necessary to perform temperature control of thenonlinear optical medium 5. The nonlinear optical medium 5 is cooleddown by the Peltier element 14 via the optical resonator block 11.

The temperature of the nonlinear optical medium 5 is detected by athermister 17 installed in the optical resonator block 11. Based on thedetected temperature of the thermister 17, electric current to theoptical resonator block 11 is controlled, and the temperature of thenonlinear optical medium 5 is controlled via the optical resonator block11.

The output of the laser beam becomes higher in recent years, and thelaser light source 2 comprises now a plurality of semiconductor lasersinstead of a single semiconductor laser. As a method in order to bundlethe laser beams emitted from a plurality of semiconductor lasers to asingle luminous flux, such method is adopted that the laser beamsemitted from the semiconductor lasers are received separately throughcorresponding optical fibers, and the optical fibers are bundledtogether to form a single cable. By this cable, the laser beams areguided to the condenser lens 3, and a single luminous flux is projected.

In the conventional example as described above, the nonlinear opticalmedium 5 is cut out in such manner that the nonlinear crystal axis ofthe nonlinear optical medium 5 is in parallel to the bottom surface ofthe recessed portion 12. Actually, however, there are errors duringcut-out operation, and the axes are not always accurately parallel toeach other. In case the required output is low or in case there is somesurplus in the output, it is not necessary to adjust the angle of thenonlinear optical medium 5 to the bottom surface of the recessed portion12. However, this does not provide sufficiently high accuracy whenhigher output efficiency is required or in case the output near thetheoretical limit is needed.

The nonlinear optical medium 5 is placed on and is closely fitted to theoptical resonator block 11 via its one surface, and a heat transfer rateof the nonlinear optical medium 5 to the optical resonator block 11 isstrongly influenced by contact condition of the surfaces.

When the angle adjusting jig 15 is removed after once angle adjustmentof the nonlinear optical medium 5 is completed, phenomenon such that thesecond harmonic output changes occurs. When the angle adjusting jig 15has been separated from the nonlinear optical medium 5, which had beenintegrated with the angle adjusting jig 15 at the angle adjustment,transfer of heat to the angle adjusting jig 15 is lost, and the pressurefrom the angle adjusting jig 15 to press the nonlinear optical medium 5is lost. As a result, changes occur in the heat transfer rate of theoptical resonator block 11, and temperature distribution (temperaturegradient) inside the nonlinear optical medium 5 is changed.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a laser oscillationapparatus, by which it is possible to align a nonlinear crystal axis ofa nonlinear optical medium with higher accuracy, to strictly satisfyphase coordinating conditions of the nonlinear crystal of the nonlinearoptical medium, and to prevent the change of output conditions after thealignment of the nonlinear crystal axis.

To attain the above object, the laser oscillation apparatus according tothe present invention comprises a laser beam emitting unit for emittingan excitation light, an optical resonance unit at least having a lasercrystal and an output mirror, a nonlinear optical medium which is placedin the optical resonance unit and which generates a second harmonic froma fundamental wave oscillated by the laser crystal, and a driving meansfor driving the laser beam emitting unit, wherein the nonlinear opticalmedium is held by a nonlinear optical medium holder, and the nonlinearoptical medium holder can be rotated with respect to axes of at leasttwo directions crossing to an optical axis passing through the opticalresonance unit. Also, the present invention provides the laseroscillation apparatus as described above, wherein a nonlinear opticalmedium for generating a sum frequency wave is used instead of thenonlinear optical medium for generating the second harmonic. Further,the present invention provides the laser oscillation apparatus asdescribed above, wherein a nonlinear optical medium for generating adifference frequency wave is used instead of the nonlinear opticalmedium for generating the second harmonic. Also, the present inventionprovides the laser oscillation apparatus as described above, wherein aPeltier element is integrally incorporated in the nonlinear opticalmedium holder. Further, the present invention provides the laseroscillation apparatus as described above, wherein the optical resonanceunit comprises an optical resonator block, the nonlinear optical mediumholder is rotatably mounted on a spherical seat of the optical resonatorblock and is tiltably mounted on the optical resonator block via thespherical seat.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an entire arrangement of an embodiment of thepresent invention;

FIG. 2 is a front view of the same;

FIG. 3 is a plan view of a laser beam emitting unit of the embodiment ofthe present invention;

FIG. 4 is a front view of the laser beam emitting unit;

FIG. 5 is a side view of the laser beam emitting unit;

FIG. 6 is a drawing to explain a fiber array unit of the embodiment ofthe present invention;

FIG. 7(A) and FIG. 7(B) each represents a drawing to explain positionaladjustment of the fiber array unit;

FIG. 8 is a drawing to show a direction of positional adjustment of thefiber array unit;

FIG. 9 is a cross-sectional view of an optical resonance unit of theembodiment of the present invention;

FIG. 10 is a perspective view of the optical resonance unit of theembodiment of the present invention;

FIG. 11 is a schematical drawing of a basic arrangement of the opticalresonance unit of a laser oscillation unit; and

FIG. 12 is a cross-sectional view of a conventional type opticalresonance unit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Description will be given below on an embodiment of the presentinvention referring to the drawings.

FIG. 1 and FIG. 2 each represents an entire arrangement of a mechanismof a laser oscillation apparatus.

A laser beam emitting unit 21 and an optical resonance unit 22 are fixedon a base 23, which serves as a heat sink. The laser beam emitting unit21 and the optical resonance unit 22 are arranged on an optical axis 10,and a lens unit 37 is disposed between the laser beam emitting unit 21and the optical resonance unit 22.

Now, description will be given on the laser beam emitting unit 21referring to FIG. 3 to FIG. 8.

A light source unit base 25 is mounted on the base 23 via a light sourcebase 24 (FIGS. 1 and 2), and a light source unit 26 is fixed on thelight source unit base 25. The light source unit 26 is provided with aplurality of semiconductor lasers 27, and the semiconductor lasers 27are disposed in a linear direction.

Opposite to the light source unit 26, a fiber array unit 28 is mountedon the light source unit base 25. The fiber array unit 28 is providedwith as many optical fibers 29 as the semiconductor lasers 27.

Now, description will be given on the fiber array unit 28.

The optical fibers 29 are sandwiched between a V-grooved base plate 32and a holding plate 33. On the V-grooved base plate 32, as manyV-grooves 34 as the optical fibers 29 are formed and the V-grooves 34run in parallel to the optical axis 10. Because the optical fibers 29are inserted and held in the V-grooves 34, the optical fibers 29 aremaintained in parallel to each other.

The optical fibers 29 held by the V-grooved base plate 32 and theholding plate 33 are extended toward the optical resonance unit 22 andare bundled in cylindrical form. Then, the optical fibers 29 areextended to the optical resonance unit 22 as a fiber cable 35, and anend surface of the fiber cable 35 is fixed in such manner as to face thecondenser lens 3.

All of the optical fibers 29 are sandwiched between the V-grooved baseplate 32 and the holding plate 33 and are integrated. To perform thepositioning for each of the optical fibers 29 with the semiconductorlaser 27, a position of the light source unit 26 should be aligned witha position of the fiber array unit 28, or positioning should beperformed either for the light source unit 26 or for the fiber arrayunit 28. In the following, description will be given on a case where thepositioning of the fiber array unit 28 can be performed.

The fiber array unit 28 is mounted on the light source unit base 25 viaa pair of wedge-shaped holders 36 a and 36 b, which are disposed at leftand right respectively and are arranged in the direction of the opticalaxis 10.

Each of the wedge-shaped holders 36 a and 36 b has a cross-section intriangular form, or more preferably in form of an isosceles triangle.The fiber array unit 28 is held between the wedge-shaped holders 36 aand 36 b in such manner that each of lateral ends of the fiber arrayunit 28 is brought into contact with the middle point on each ofinclined surfaces of the wedge-shaped holders 36 a and 36 b, which arefacing to each other.

As shown in FIG. 8, by adjusting the positions of the wedge-shapedholders 36 a and 36 b, angular adjustment of the fiber array unit 28 canbe performed in three angles: an angle θ around θ axis running in avertical direction, a rotating angle α around the optical axis 10, andan angle γ around γ axis, which runs perpendicularly to both the opticalaxis 10 and θ axis.

Now, detailed description will be given referring to FIGS. 6, 7 and 8.

First, it is assumed that the wedge-shaped holders 36 a and 36 b aredisposed in parallel to each other along the optical axis 10.

In case the angle α is adjusted, e.g. in case the angle α is adjusted ina clockwise direction in FIG. 6, if the right end of the fiber arrayunit 28 is moved down along the inclined surface of the wedge-shapedholder 36 b, the left end of the fiber array unit 28 is displaced upwardalong the inclined surface of the wedge-shaped holder 36 a to an extentequal to the extent of downward displacement of the right end. As aresult, the angle α is adjusted. As described above, in case each of thewedge-shaped holders 36 a and 36 b have a cross-section of the sameisosceles triangle, an amount of displacement at left is equal to anamount of displacement at right.

In case the angle γ is adjusted, e.g. in case the angle γ is adjusted ina counterclockwise direction in FIG. 7(B), a distance “a” between thewedge-shaped holders 36 a and 36 b on a side closer to the light sourceunit 26 is widened compared with a distance “b” on a side closer to theoptical resonance unit 22 in FIG. 7(A). With respect to the widening ofthe distance between the wedge-shaped holders 36 a and 36 b, each pointon the fiber array unit 28 is displaced downward in such extent as tomatch a tangent component of a tilt angle on the inclined surface ofeach of the wedge-shaped holders 36 a and 36 b. That is, the angle γ isadjusted.

In case the angle θ is adjusted, the fiber array unit 28 should berotated around the θ axis while maintaining the relationship of thewedge-shaped holders 36 a and 36 b.

In the above explanation, the wedge-shaped holders 36 a and 36 b aredisposed in parallel to the optical axis 10, while it is needless to saythat the adjustment can also be made in the same manner when the holdersare arranged in a direction perpendicular to the optical axis 10.

When the adjustment of the fiber array unit 28 has been completed, thewedge-shaped holders 36 a and 36 b are fixed by adequate means such asbonding on the light source unit base 25.

A rod lens 31 is mounted in parallel to end surfaces of the opticalfibers 29 of the fiber array unit 28 by bonding. As a method of bonding,UV-bonding not causing thermal distortion is used. As the rod lens 31, acore portion of the optical fiber 29 is cut off in an adequate lengthand is used. The rod lens 31 can effectively converge the laser beams toeach of the optical fibers 29, and it can be manufactured in easy mannerand is not expensive. It is of course satisfactory that a rod lenshaving the same diameter as a diameter of the optical fiber 29 may beseparately manufactured and used.

By bonding the rod lens 31 to the fiber array unit 28, optical relationbetween the rod lens 31 and the fiber array unit 28 can be maintainedand integrally adjusted, and this facilitates the execution of theadjusting procedure.

A luminous flux of the laser beam emitted from each of the semiconductorlasers 27 has an elliptic cross-section. When the light beams areconverged in one direction by the rod lens 31, it is possible to projecta luminous flux with a cross-section closer to a true circle to theoptical fiber 29, and this reduces the loss of the laser beams when thebeams are projected to the optical fibers 29. The position of the fiberarray unit 28 can be adjusted completely in directions of the three axeswith respect to the light source unit 26, and this reduces the loss dueto giving and receiving of the laser beams between the light source unit26 and the fiber array unit 28.

Description will be given on the optical resonance unit 22 referringFIG. 9 and FIG. 10. In FIG. 9 and FIG. 10, the same component in FIG. 11or FIG. 12 is referred by the same symbol.

The lens unit 37 provided with the condenser lens 3 is mounted on a sideof the optical resonance unit 22 closer to the laser beam emitting unit21. On the lens unit 37, a tip of the fiber cable 35 is mounted via acable holder 38. An optical axis of the fiber cable 35 is aligned withan optical axis of the condenser lens 3.

An optical resonator block 39 is fixed on the base 23. A recessedportion 12 is formed from above on the optical resonator block 39, andan optical path hole 13 passing through the recessed portion 12 isformed with its axis aligned with the optical axis 10. A laser crystal 4is provided on a portion of the optical path hole 13 closer to thecondenser lens 3. A first dielectric reflection film 7 is formed on anend surface of the laser crystal 4 closer to the condenser lens 3. Anoutput mirror 6 is arranged on an exit end of the optical path hole 13,and a second dielectric reflection film 8 is formed on a concavereflection surface of the output mirror 6.

The upper surface of the optical resonator block 39 runs in parallel tothe surface of the base 23 where the optical resonator block 39 ismounted, and a convex cylindrical curved seat 41 is fixed on the uppersurface of the optical resonator block 39. The convex cylindrical curvedseat 41 is designed in such shape that a part of a cylinder is cut offby a plane running in parallel to the central line of the cylinder, andit is mounted in a direction perpendicular to the optical axis 10.

A concave cylindrical curved seat 42 is slidably engaged with the convexcylindrical curved seat 41, and a nonlinear optical medium holder 43 ismounted on the concave cylindrical curved seat 42. The center of acylindrical curved surface of each of the convex cylindrical curved seat41 and the concave cylindrical curved seat 42 is on the optical axis 10.

As the nonlinear optical medium holder 43, a material having high heattransfer property or, preferably, having lower thermal expansioncoefficient is used. The nonlinear optical medium holder 43 is rotatablyengaged with the concave cylindrical curved seat 42 and the non linearoptical medium holder 43 is passing through the convex cylindricalcurved seat 41 with a certain clearance or play. A lower portion 43 b ofa flange 43 a is accommodated in the recessed portion 12. An upper endof the nonlinear optical medium holder 43 is exposed above the concavecylindrical curved seat 42, and the exposing portion is the flange 43 a.This is used as a knob when the nonlinear optical medium holder 43 is tobe rotated. On the upper surface of the flange 43 a, slit grooves (notshown) may be formed to enable the adjustment using tools.

At a position on the lower portion 43 b where the optical axis 10 passesthrough, a nonlinear optical medium 5 is provided in such manner that 4surfaces except surfaces running perpendicularly to the optical axis 10are closely fitted to the lower portion 43 b and a Peltier element 14 ismounted on upper side of the nonlinear optical medium 5. The Peltierelement 14 is provided in such manner that it is squeezed at the middleof the lower portion 43 b or it is engaged in a mounting hole formed onthe lower portion 43 b. At least, two surfaces on upper and lower sidesare closely fitted to the lower portion 43 b.

On the nonlinear optical medium holder 43, wiring holes 44 running inparallel to a central axis are formed from above and extended until theholes reach the Peltier element 14. Lead wires 46 guided from atemperature controller 45 are connected to the Peltier element 14through the wiring holes 44. A thermister 17 is arranged at apredetermined position on the nonlinear optical medium holder 43, ormore preferably, at a position closer to the nonlinear optical medium 5.

The nonlinear optical medium holder 43 can be rotated around the axis ofthe nonlinear optical medium holder 43 integrally with the Peltierelement 14 and the nonlinear optical medium 5. Further, it can also berotated around γ axis, which passes through the optical axis 10.

Deviation of the nonlinear crystal axis of the nonlinear optical medium5 from the optical axis 10 exerts influence on the output of the laseroscillation apparatus. The sensitivity to the deviation of the nonlinearcrystal axis of the nonlinear optical medium 5 from the optical axis 10differs according to the axial direction. In the above embodiment, theadjustment on the angle α is omitted, at which the influence of thedeviation of the nonlinear crystal axis is at the lowest. Thus,efficiency can be sufficiently increased only by making it possible toadjust the angle γ in addition to the angle θ. By omitting theadjustment for the angle α, it is possible to simplify the structure andthe adjusting procedure. If the efficiency is improved more, a convexcurved seat and a concave curved seat having spherical curved surfacesmay be used instead of the convex cylindrical curved seat 41 and theconcave cylindrical curved seat 42. Then, the nonlinear optical mediumholder 43 can be also rotated around α axis. Thus, it is possible toperform angular adjustment in three directions of the angles θ, γ and α,and the nonlinear crystal axis can be aligned more perfectly.

As described above, the adjustment of the angle γ can also be achieved,and it is possible to adjust including the correction of error duringthe cut-out operation of the nonlinear optical medium 5. This makes itpossible to strictly satisfy phase coordinating condition of thenonlinear crystal of the nonlinear optical medium 5. As a result, outputefficiency can be increased up to near the output of the theoreticallimit.

The nonlinear optical medium 5 is engaged in the lower portion 43 b, andthe Peltier element 14 is integrally incorporated in the lower portion43 b. Heat transfer area of the nonlinear optical medium holder 43between the nonlinear optical medium 5 and the Peltier element 14 isbig. Heat transfer surface contact pressure can be determined at thetime of incorporation, and a stable heat transfer rate can be ensured.Therefore, cooling effect on the nonlinear optical medium 5 by thePeltier element 14 is high. The nonlinear optical medium 5 and thePeltier element 14 are integrated with the nonlinear optical mediumholder 43, and holding conditions of the nonlinear optical medium 5 andthe Peltier element 14 are not changed after the adjustment.Accordingly, no change occurs in the cooling condition due to thePeltier element 14 after the adjustment.

Temperature control of the nonlinear optical medium 5 can be easilyadjusted, and the accuracy during operation is also improved.

In the above embodiment, a nonlinear optical medium generating a secondharmonic is used, while it is needless to say that the same results andeffects can be obtained when a nonlinear optical medium generating a sumfrequency wave or a difference frequency wave is used.

As described above, energy utilization efficiency is improved by opticalaxis alignment between the light source unit 26 and the fiber array unit28 and by accurate alignment of the nonlinear crystal axis of thenonlinear optical medium 5 with the optical axis 10, while energyutilization efficiency can also be improved by reducing reflection lossof the optical system.

Reflection loss on an entrance surface and an exit surface when a laserbeam enters and exits an optical member is about 4% on each of thesurfaces. This reflection loss occurs at the same number as the numberof optical members. This is a considerable loss if it is seen from theentire optical system.

Therefore, AR coating is provided on end surfaces of the rod lens 31 andthe optical fibers 29 in the above embodiment.

As the means for performing AR coating, there are the following methods:to directly form a dielectric thin film on the end surface by vacuumdeposition or by ion plating; or, to mount a cover glass with AR coatingon the end surface, etc. Because the rod lens 31 and the optical fibers29 are made of synthetic resin and these are thin, each having adiameter of several tens of μm to 500 μm. In this respect, the followingmethod is adopted to avoid the damage of the material.

The end surface of the optical fiber is immersed into a solution of afluorine type compound, and after it is taken up, it is dried andhardened. In this case, concentration of the solution and the take-upspeed should be selected to have such refractive index and coatingthickness that AR effects can be obtained in the wavelength range inuse. By this method, it is possible to form AR coating very easily withreflectivity of 1% or less—not only on the end surface of the opticalfiber but also on a cylindrical surface of the optical fiber. In thepast, for providing AR coating to a cylindrical surface of a very smallcylindrical lens, etc., special type tools have been needed in a vacuumapparatus. By the method to immerse the end surface of the optical fiberinto the solution of the fluorine type compound and to dry and hardenafter it is taken up, the procedure can be easily performed underatmospheric pressure. Further, by adjusting the take-up speed or thesolution concentration, it is possible to obtain multi-coating withreflectivity of 0.5% or less by repeating the procedure by two or moretimes.

Naturally, it is possible to control the reflectivity by changing theconditions of the solution (concentration; refractive index). Forinstance, it is possible to obtain a reflection film with reflectivityof 50% or 80%.

As a result, reflection loss as high as 8% to 10% on the entrance andexit end surfaces can be improved, and energy utilization efficiency isincreased. Thus, the trouble such as damage of the optical system due tothe reflection light can be extensively reduced.

The laser oscillation apparatus according to the present inventioncomprises a laser beam emitting unit for emitting an excitation light,an optical resonance unit at least having at least a laser crystal andan output mirror, a nonlinear optical medium which is placed in theoptical resonance unit and for generates a second harmonic from afundamental wave oscillated by the laser crystal, and a driving meansfor driving the laser beam emitting unit, wherein the nonlinear opticalmedium is held by a nonlinear optical medium holder, and the nonlinearoptical medium holder can be rotated with respect to axes of at leasttwo directions crossing to an optical axis passing through the opticalresonance unit. This makes it possible to strictly satisfy theconditions of phase coordination of the nonlinear crystal of thenonlinear optical medium and to improve output efficiency.

Because a Peltier element is integrally incorporated in the nonlinearoptical medium holder, accuracy of temperature control can be increased,and output stability is also improved.

What is claimed is:
 1. A laser oscillation apparatus, comprising a laserbeam emitting unit for emitting an excitation light, an opticalresonance unit at least having a laser crystal and an output mirror, anonlinear optical medium which is placed in said optical resonance unitand which generates a second harmonic from a fundamental wave oscillatedby said laser crystal, and a driving means for driving said laser beamemitting unit, wherein said nonlinear optical medium is held by anonlinear optical medium holder, said optical resonance unit has aspherical seat for rotatably supporting said nonlinear optical mediumholder, and said nonlinear optical medium holder can be adjusted bybeing rotated with respect to axes of at least two directions crossingto an optical axis passing through said optical resonance unit.
 2. Alaser oscillation apparatus according to claim 1, wherein a Peltierelement is integrally incorporated in said nonlinear optical mediumholder.
 3. A laser oscillation apparatus according to claim 1, whereinsaid optical resonance unit comprises an optical resonator block forholding said laser crystal, said spherical seat is provided on saidoptical resonator block, and said nonlinear optical medium holder isrotatably mounted on said spherical seat and is tiltably mounted on saidoptical resonator block via said spherical seat.
 4. A laser oscillationapparatus according to claim 1, wherein said nonlinear optical mediumholder has a flange portion and a lower portion for holding saidnonlinear optical medium, and a cooling member is provided at said lowerportion.
 5. A laser oscillation apparatus according to claim 1, whereinsaid nonlinear optical medium is mounted in said nonlinear opticalmedium holder in such a manner that four surfaces except surfacesrunning perpendicularly to an optical axis of said nonlinear opticalmedium are closely fitted to said nonlinear optical medium holder.